Cranes of the general type to which this invention relates are disclosed, for example, in U.S. Pat. Nos. 3,842,984; 4,258,852 and 4,540,097.
Such a crane has an earthborne base that is usually mounted on wheels or crawler treads, and has a platform mounted on the base for rotation about a vertical axis. Pivoted to the platform for swinging substantially in a plane that contains the vertical axis is a boom that normally extends up from the base at a forward inclination to that axis. A load line passing over the upper end of the boom has one end depending from the boom to be connectable with a load and has its opposite end connected with a winch on the platform.
Conventionally the platform has had a tail-like rearwardly projecting portion, to which was attached a main counterweight that offset the forward tilting force exerted by the boom and by any light to moderately heavy load the crane was hoisting.
When such a crane was used for lifting a very heavy load, it was often provided with a mast that had a pivot connection with the platform, which connection was behind the boom connection and allowed the mast to swing relative to the platform in substantially the same plane that contained the swinging of the boom. A tension line connected between the upper ends of the boom and the mast tended to make them swing in unison and normally positioned the mast to project up from the platform at a rearward inclination to the vertical axis. A line attached to the top of the mast was connected with an auxiliary counterweight that was normally ground-supported. When a heavy load was being hoisted, the boom would swing forwardly and downwardly in reaction to the lift force exerted on the load, and through the tension line the mast was correspondingly swung forwardly and upwardly. The swinging of the mast lifted the auxiliary counterweight off of the ground, so that the full weight of the auxiliary counterweight, in addition to that of the main counterweight, was applied to offsetting the tilting force that the load exerted upon the boom. A gantry tructure fixed on the rear part of the platform had a lost motion connection with the mast that defined the forward limit of swinging motion of the boom relative to the platform.
For transport from one job site to another, a crane of the type here under consideration is adapted to be partially disassembled into units that comply with size and weight limitations prescribed for highway vehicles. Hence a mobile crane should be as compact, as light, and as easy to assemble and disassemble as is consistent with its hoisting capacity. It is also desirable that such a crane have the smallest possible tail swing, that is, that its upper structure, including counterweights, project the least possible distance behind the vertical axis so that it can operate in a relatively restricted space without interference from obstacles around its rear.
Apparently it has not heretofore been obvious that the tall, bulky and relatively heavy mast considered necessary for equipping a mobile crane to hoist relatively heavy loads could be replaced for all but the very heaviest loads with a lower, lighter and more compact structure that would reduce tail swing, would be easier to assemble and disassemble, and would be more convenient and economical to transport.